Under the EPA's Clean Air Mercury Rule, coal fired power plants are required to drastically reduce the amount of mercury (Hg) emissions within the next several years. One of the technologies under consideration for removal of Hg is the use of chemically treated (brominated) activated carbon. However, currently used activated carbon is not resistant to SO3 poisoning. It has been postulated that there are possible mechanisms of interaction of sulfur oxides with activated carbon. It is said that SO2 and SO3 compete with mercury for Lewis base sites on the surface of activated carbon. Further, the concentration of SO2 (several pph to 1000 ppm) and SO3 (1-10 s ppm) are significantly greater than mercury concentration (˜1 ppb). Additionally, SOx molecules are kinetically and thermodynamically favored over mercury due to their strong binding energy to activated carbon. Activated carbon acts as a catalyst during the reaction between SO3 and water to form sulfuric acid. SO3 can also react with surface oxygen to form H2SO4. One of the possible solutions suggested others is to co-inject a basic sorbent with activated carbon. However, injection of two sorbents for the removal of mercury can significantly increase the cost of mercury removal.
Coal fired power plants constitute ˜52% of the total electricity produced in United States. As the demand for electricity increases, Utility companies are increasing the generating capacity as well. Additionally, many of the current nuclear plants will be “retired” in the first quarter of the 21st century. Due to poor public support for nuclear energy, these nuclear plants are likely to be replaced by coal fired plants. At the current consumption rate, it is estimated that the world has ˜1500 years of coal reserves. This leads to a steady increase in amount of coal consumed in the world and in the US. This implies that the mercury emission issue associated with coal-fired power plants needs to be resolved in the long run.
An estimated total of 48 tons of mercury is emitted every year in the US from coal-fired power plants, which is ⅓rd of the total mercury emissions per year in the US. On a worldwide scale, this is a much larger issue, since countries such as China and India are using increasing amounts of energy derived from fossil fuels. Under the government's “Clear Skies Initiative”, the target is to reduce mercury emission by about 45% by 2010, and about 70% by 2018. New technologies will need to be developed to reach these targets. According to the DoE, the market penetration for mercury emission reduction technologies is an estimated 320,000 megawatts. In order to achieve the target reduction by 2018, the additional annual cost for energy generation will be $2 to $6 billion per year, if the existing activated carbon technology is used (current estimate is $18,000-$131,000 per pound of mercury removal, using activated carbon).
Currently various types of activated carbons are being extensively studied for mercury removal from flue gas. DOE/NETL has carried out several field tests of activated carbons due to their high removal efficiency. Three prominent brands of activated carbons which have been tested in the field are NORIT Americas (Darco® Hg-LH), Alstom Power Plant Laboratories (Mer-Clean™), and Sorbent Technologies Corporation (B-PAC™). Results indicated that activated carbon consistently performed well in mercury removal, on a full-scale test. However, secondary pollution (bromine), corrosion from bromine and SO3 resistance is still an issue, affecting their overall performance.
A study tested the performance of a commercial sorbent Darco Hg-LH, in the presence and absence of SO3, at a 630 MW power plant. The mercury removal efficiency of Darco Hg-LH reduced from 75-90% mercury removal in the absence of SO3, to 50-60% mercury removal in the presence of 5.4 ppm SO3. The mercury removal efficiency reduced further to 33% in the presence of 10.7 ppm SO3.
Another media which is used to remove mercury from flue gas is based on “clay”, and is manufactured by Amended Silicates. However when the performance of this media was compared with various types of activated carbon sorbents the amended silicate media did not perform as well as activated carbon. Others used a fluidized bed of zeolite and activated carbon for the removal of organics and metals form gas streams. Zeolites are aluminosilicate materials that are extensively used as adsorbents for gas separation and purification, and they are also used as ion-exchange media for water treatment and purification. Zeolites have “open” crystal structures, constructed from tetrahedra (TO4, where T=Si, Al). It has been observed that the removal efficiency of metals present in gases by activated carbon is higher than that of zeolite, and the temperature only slightly influences the removal efficiency. A study tested treated Zeolite and observed 63% mercury removal efficiency.
U.S. Pat. No. 6,610,263 is directed to the use of high surface area MnOx to remove Hg. It is claimed that it has the capability to remove 99% of elemental Hg and 94% of the total mercury content in flue gas. However, the cost is likely to be a concern for using this media in practical applications.
Biswas et-al [T. M. Owens and P. Biswas, J. Air & Waste Manage. Assoc., n46, 1996, p 530] have developed a gas-phase sorbent precursor method, where a high surface area agglomerated sorbent oxide particle is produced in situ in the combustor. These sorbents are stable at elevated temperatures and provide a surface of metallic vapors (for condensation) and reaction. They used titanium isopropoxide as precursor, which decomposed at elevated temperature and formed particles of titania. Hg vapors were found to condense on these particles in the presence of UV radiation which helps in the oxidation of mercury vapors and formation of a strong bond between mercury and titania. They [P. Biswas and M. Zachariah, “In situ immobilization of lead species in combustion environments by injection of gas phase silica sorbent precursors”, Env. Sci. & Tech., v31, n9, 1997, p 2455] also used silica precursors for the removal of lead from flue gas, and were able to get 80-90% lead removal efficiency. The removal efficiency was found to be a function of the gas temperature. Additionally, the efficiency was observed to decrease with increase in temperature.
Another group have shown the feasibility of using a fluidized bed for the removal of metals, such as lead, from flue gas. They used limestone, bentonite, and alumina as sorbents, and observed that the effectiveness of the fluidized bed depends on sorbent species, sorbent particle size, the amount of sorbent used, metal to sorbent ratio, metal concentration in the waste, air velocity, and temperature. Smaller particles showed better efficiency compared to larger particles (particle range 400-700 μm). In case of limestone, it increased from 60% to 70% when the particle size was decreased from 700 to 500 μm, all other conditions remaining same. The sorbents showed better efficiency at lower temperatures (˜750° C. vs. ˜900° C.). This is because at higher temperatures, the vapor pressure is high, so more metal escapes as vapor.
Still others have used zeolite materials for the removal of mercury by duct injection. They were able to get between 45 and 92% metal removal depending upon the amount of sorbent injected and the type of sorbent. In the case of zeolites, there was no effect of temperature on the removal efficiency.
Gullet et-al [B. Gullet and K. Raghunathan, “Reduction of coal based metal emissions by furnace sorbent injection”, Energy & Fuels, v8, 1994, p 1068] demonstrated the feasibility of using oxide minerals such as limestone, kaolinite, and bauxite as sorbents for toxic metal removal, by injecting them through the burner. They were able to get reduction in submicron size metal particles of antimony, arsenic, mercury, and selenium by hydrated lime and limestone.